The present invention relates generally to a communications systems, and more specifically to a local carrier frequency control technique to establish synchronization with a transmitted carrier.
Satellite communications between very small aperture terminals operate on the convolutional coding scheme to take advantage of its low-cost but powerful forward error correcting capability, instead of the expensive pilot signaling scheme, due to the demanding cost effectiveness. This results in a VSAT design that incorporates automatic frequency tracking ability in order to operate with a satellite whose transmit frequency is subject to change under hostile environment.
Conventional VSAT's employ coherent detection for demodulating a received M-ary PSK (phase-shift keyed) convolutional codes. The received PSK signal is mixed with local carriers to produce I- and Q-channel signals, which are coupled to a convolutional decoder. Phase difference between the demodulated signals and the local carriers is detected and is used to control the carriers' phase to establish carrier synchronization. Synchronization is also established between the frequency of the local carriers and the symbol rate of the convolutional codes by detecting the error rate of the I- and Q-channel signals and controlling the carrier frequency so that the error rate is reduced below a specified value. A replica of the transmitter's convolutional encoder is coupled to the convolutional decoder and a correlator is provided for determining the correlation between the output of the convolutional encoder and one input of the convolutional decoder. Since the output of the convolutional encoder is a replica of the original codeword, the correlator produces a signal which increases in amplitude with the error rate of the signal at the input of the convolutional decoder. A maximum detector is connected to the correlator to control the phase of the input to the convolutional decoder when the error rate exceeds a specified value.
A coherent M-ary PSK detector is described in European patent application no. 0 369 406 as a solution to this problem. According to this prior art, a convolutional decoder corrects errors contained in demodulated I- and Q-channel baseband signals during its decoding process. The error rate of the output of the convolutional decoder is monitored and an in-sync detect signal is generated when the error rate is smaller than a specified threshold as an indication that the convolutional decoder is synchronized with the I- and Q-channel signals. A series of different VCO control voltages are generated as corresponding to frequency candidates in response to the in-sync detect signal to permit the PSK detector to synchronize successively with the baseband signals at the candidate frequencies. The power levels of the information and noise components of the baseband signals are further detected and signal-to-noise ratios are successively derived from the detected power levels. A maximum value of the signal-to-noise ratios is then detected and one of the candidate VCO control voltages that corresponds to it is adopted and maintained as a valid VCO control voltage.
However, one shortcoming of this prior art is that, since signal-to-noise ratios are measured at all points below the decision threshold of the error rate determination, it takes long to establish synchronism. Another disadvantage is that, since the signal-to-noise decreases with the ratio of energy-per-bit to noise density (E.sub.b /N.sub.o), difficulty arises in determining whether the PSK detector is properly synchronized during low E.sub.b /N.sub.o ratios.